Neuronal activity in the ventral tegmental area (VTA) of brain is thought to be involved in alcohol preference and reward mechanisms by regulating mesolimbic dopamine (DA) release. We are studying the excitability mechanisms in freshly isolated VTA neurons from rat brain using the whole-cell patch-clamp recording method combined with single-cell RT-PCR analysis. VTA prinicpal neurons, which were identified as dopaminergic, exhibited low firing frequency, a wide action potential (AP) half-width, and a calbindin-positive and palvalbumin-negative Ca2+-binding protein expression pattern. The mean firing frequency of VTA principal neurons was 2.6 Hz and this positively correlated with the afterhyperpolarization (AHP) slope, but not the AHP amplitude. Based on firing pattern there were two distinct subpopulations of VTA principal neurons: an irregular-type (52%) and a tonic-type (48%). The irregular and tonic firing patterns were also recorded in cell-attached recording. The irregular-type VTA principal neurons exhibited a longer AP half-width and a smaller APH amplitude, compared with the tonic-type VTA principal neurons. Apamin-sensitive Ca2+-activated small conductance K+ (SK) channels contributed to the regular firing pattern of the tonic-type neurons, but contributed little to the firing pattern of the irregular-type neurons. In voltage-clamp tail current analysis, apamin-sensitive AHP current density in the tonic-type neurons was 6-fold larger than that of the irregular-type neurons. Apamin-sensitive tail current deactivation in the tonic-type neurons was best fitted with a single exponential curve (53 ms). Our observations suggest that SK channels in the AHP differently contribute to the two types of spontaneous firing in VTA principal neurons. Experiments are also in progress to elucidate the cellular mechanisms of alcohol and neuroactive substance action on excitability mechanisms in VTA and other types of neurons. These studies hold the promise that such investigations on excitability mechanisms will advance our knowledge of alcohol and neuroactive substance action in the nervous system and provide a foundation for understanding the role of excitability mechanisms in alcohol abuse and alcoholism.